CHAPTER XVI. 
SYMMETRY OF THE SPOROPHYTE. 
Amonc plants at large various types of symmetry are recognised. The 
most simple form is the sphere, a type of symmetry without polarity, that 
is, having no distinction of apex and base: it is the usual initial form of 
the individual, when it is first delimited as the zygote or the spore. Occa- 
sionally this form may be retained to maturity, as in the sporogonium of 
#iccia: but in the vast majority of plants polarity appears early in the 
individual life, usually with growth localised in relation to it. The body 
thus produced may develop variously as regards an imaginary axis of 
construction, which passes between the two poles. Three types of 
symmetry are usually distinguished where polarity exists: (1) the radial 
construction, where the development is equal in all directions round the 
imaginary axis: (2) the d¢symmetric or bilateral, in which the construction 
is flattened equally on both sides: and (3) the dorsiventral, where the 
construction is also flattened, but not equally on both sides, the result 
being two faces which differ obviously from one another in form, and 
usually also in inner structure. These types of symmetry may, as a rule, 
be related to the external conditions under which the parts are developed : 
thus orthotropous, or vertical parts are almost always radial or bilateral ; 
while plagiotropous, that is oblique or horizontal parts, are commonly 
dorsiventral, or occasionally bilateral. Dorsiventrality of the shoot, where 
it exists, has usually some evident relation to the external conditions of 
life, such as the incidence of unequal lighting, or oblique disposition to 
the action of gravity: and it may also be seen in lateral branches to be 
connected with the relation of the part in question to the chief shoot 
which bears it. In some cases it may arise from inner causes,! but 
investigation has shown that dorsiventrality of the shoot is usually to be 
referred to some external determining influence. 
1A striking example of this is brought forward by Willis, in the Podostemaceae, 
where dorsiventrality appears in erect and anemophilous flowers. He suggests that this 
condition has been forced upon them, without reference to any advantage, by the steadily 
increasing dorsiventrality of the vegetative system (Axnals of Botany, 1902, p. 593). 
